Google Sidewiki is a new browser plug-in that adds a universal commenting system to the web, allowing users to comment and read other people's comments on any page on the internet. It's not a new idea, but, well, it's Google.
Sidewiki installs alongside the Google Toolbar, so it works with both Internet Explorer and Firefox. Google also says that they're "working on making it available in Google Chrome and elsewhere too." (Hopefully they'll make it an extension for Chrome, too, rather than forcing it down user's throats.)
After you install Google Toolbar with Sidewiki and restart your browser, you'll end up on a landing page that illustrates how the tool works and encourages you to write your first entry. (Incidentally, when you comment on a page using Sidewiki, that comment will show up in your Google Profile page as well—like this.) Comments on the internet aren't exactly known for their quality (Google-owned YouTube is notorious for having some of the worst, most inane comments on the internet), but Google's aiming to address that with Sidewiki: